1. Field of the Invention
The invention relates to a diffuser having a scattering structure for a wavefront source, to a corresponding wavefront source comprising, in addition to the diffuser, a wavefront formation structure in the beam path after or at the level of the scattering structure, to an associated wavefront sensor and an associated projection exposure apparatus.
2. Description of the Related Art
A wavefront source of this type is disclosed in the published patent application DE 101 09 929 A1 as part of a device for the wavefront measurement of optical systems by means of shearing interferometry, in particular of a projection objective of a microlithography projection exposure apparatus. A two-dimensional multihole mask, also referred to as a coherence mask, preferably functions as the wavefront formation structure there. The spatial structure of the wavefront source serves for shaping the spatial coherence of the wavefront. Furthermore, it is also known from this document that the wavefront formation structure may be fitted on a ground-glass screen or a ground-glass screen may be disposed upstream of it in order to control the spatial coherence, since the illumination of the wavefront formation structure should ideally be as incoherent as possible. In addition, a refractive or diffractive focusing element operating in transmission may be provided in front of the wavefront formation structure in order to adapt the illumination aperture to the aperture diaphragm of the measured projection projective. The ground-glass screen may lie in front of or behind the focusing element.
Aberrations of a microlithographic projection objective and of other high-resolution optical imaging systems can be determined very precisely by means of the interferometric wavefront measurement mentioned. The use of the wavefront source, also referred to as a wavefront module or source module, makes it possible to utilize the illumination system part of the projection exposure apparatus or of some other measured optical imaging system for this interferometric aberration determination as well. It is advantageous for the wavefront source to be configured in such a way that it can be integrated into a microlithography projection exposure apparatus in a simple manner in order to be able to check the imaging quality of the projection objective not only prior to its installation in the projection exposure apparatus but also afterward at its place of use from time to time.
In the earlier German patent application 102 17 242.0, the priority of which is claimed by the present application and the entire scope of which is hereby incorporated by reference, it is proposed, as a developing measure, to arrange the wavefront formation structure besides a useful pattern, which is to be imaged e.g. on to a wafer as intended by the projection objective, on a common substrate. This enables an aberration determination and thus also as necessary an aberration control or aberration regulation during normal exposure operation.
For the measurement of optical systems with the wavefront provided by the wavefront source, as is known, in addition to the abovementioned shearing interferometry, other interferometric methods are also possible, but also non-interferometric methods, such as the Shack-Hartmann method or Hartmann method. In the latter case, the wavefront formation structure typically comprises an individual so-called pinhole or a one- or two-dimensional arrangement of a plurality of such pinholes, that is to say openings having a very small diameter.
Especially for the field of use of microlithography, in order to obtain very fine wafer structures, the trend is toward ever shorter exposure wavelengths in the UV range below 200 nm and particularly in the EUV range. Accordingly, there is a demand for wavefront sources that are able to supply in this wavelength range a wavefront that can be used to interferometrically measure a projection objective sufficiently accurately with regard to aberrations. One difficulty in this context is that customary illumination systems such as are used in microlithography projection exposure apparatuses in order to supply the desired UV or EUV exposure radiation have a certain pupil parceling and, moreover, do not fill the entire numerical aperture of the projection objective.
Therefore, the technical problems on which the invention is based include providing a wavefront source of the type mentioned in the introduction which is able to provide the wavefront radiation across the beam cross-section of interest comparatively homogeneously and over the entire numerical aperture of the measured optical system with comparatively high effectiveness also for UV and in particular EUV systems, and also providing a diffuser that can be used for such a wavefront source, providing a wavefront sensor equipped with such a wavefront source, and providing a projection exposure apparatus containing such a sensor.